1. Field of the Invention
The present invention relates to an information storage medium comprising plural information storage layers in a single disk-shaped information storage medium, an information reproducing method for reproducing data in sector units from the information storage medium, and an information reproducing apparatus implementing the information reproducing method.
2. Description of the Prior Art
Conventional optical disks have only one recording layer, and no consideration has been given for optical disks having plural recording layers. Magnetic storage media, however, typically have plural recording layers on each magnetic disk. The structure of such a magnetic storage media is shown in FIG. 9.
A magnetic disk typically has plural disk-shaped magnetic storage media D1 and D2, and magnetic read/write heads M1, M2, M3 and M4 for four recording surfaces. The magnetic read/write heads M1, M2, M3 and M4 are provided at the end of swing arms A1, A2, A3 and A4 which are rotated simultaneously by the stepping motor. This makes it possible to change the read/write recording surface by simply selecting the appropriate magnetic head. Plural concentric tracks are formed on each recording surface, and each track is divided into plural sectors. Each of these sectors typically has a 512-byte to 2048-byte capacity, and is used as the data recording unit. An address comprising the track number and sector number (also referred to as sector address) is written to the beginning of each sector. The magnetic disk drive depends on this address information to position the magnetic head. Track numbers are assigned in ascending order from the outside circumference to the inside circumference.
On a conventional optical disk, however, the recording track is formed as a spiral groove rather than concentric grooves. Except that the track shape is spiral, the track numbers and sector numbers of optical disk media standardized for data processing (e.g., 90 mm magneto-optical disks conforming to ISO-10090) are assigned in the same manner as on a magnetic disk.
The sector addresses on optical disk media developed first for audio storage and later adapted for data processing applications, i.e., CD-ROMs, are expressed in minutes, seconds, and frames.
To maximize the disk storage capacity of a CD-ROM or other optical disk, the recording density is constant across the entire disk surface. The disk is also driven with constant linear velocity (CLV) control to assure that a constant data quantity is reproduced per unit of time. CLV drive rotates the disk at a variable speed depending upon the radial disk position so that the beam spot focused on the disk by the optical head scans a constant distance per unit of time on the disk. Disks containing a constant recording density across the entire disk surface are therefore also known as CLV disks.
The sector arrangement on a CLV disk is shown in FIG. 10. Each fan-shaped block in FIG. 10 is a sector. The sectors are contiguously connected in a spiral pattern. Because the recording density is constant, every sector is the same size (capacity) from an inside to an outside circumference.
The internal structure of each sector is shown in FIG. 11. Each sector thus comprises a header containing the address uniquely identifying the sector, a data block to which user data is recorded, and an error correction code (ECC) block to which is recorded a code used for error correction during reproduction.
Advances in moving picture compression technologies in recent years have also made it possible to record substantially theater-quality moving pictures to a single optical disk. These disks are known as Digital Video Disks (DVD).
A single DVD can store approximately 135 minutes of high-quality moving pictures. Obviously, however, not all video sources are approximately 135 minutes long. It has therefore been proposed that the storage capacity could be approximately doubled by forming two recording layers on a single optical disk. The principle of reproducing data from a dual recording layer optical disk is shown in FIG. 12 and described below.
Strings of pits and lands are formed in a transparent substrate, which is then coated with aluminum, to form each recording layer. A transparent photosetting resin is injected between the first and second recording layers. The thickness of the aluminum on the first recording layer is adjusted to reflect half and pass half of the light incident thereon. The thickness of the aluminum on the second recording layer is adjusted to reflect all of the light incident thereon. The beam spot (focusing point) of the laser beam can be focused on the aluminum of the first or second recording layer by moving the objective lens that focuses the laser beam closer to or away from the optical disk.
The recording layers of the DVD medium are described below. As with conventional optical disks and magnetic disks, information is divided into sector units for recording to a DVD medium. The DVD sector arrangement of each recording layer is also like that of the CLV disk shown in FIG. 10. The internal structure of each sector is also the same as that of a conventional information storage medium as shown in FIG. 11.
FIGS. 13A, 13B, 13C and 13D show the spiral grooves of a conventional information storage medium having two recording layers as described above, the rotational velocity, and the reproduction direction. FIG. 13A shows the spiral groove pattern on the first layer, FIG. 13B shows the spiral groove pattern on the second layer, FIG. 13C shows the rotational velocity of the disk, and FIG. 13D shows the reproduction direction. User data is recorded to the data blocks of the first and second layers as shown in FIG. 13D. The sector address is also recorded to the lead-in and lead-out areas (shown shaded in the FIG. 13D) so that the current position can be determined when the head overruns the data block.
When the information storage medium is rotated clockwise, both first and second recording layers are reproduced from the inside circumference to the outside circumference. The rotational velocity of the information storage medium is also inversely proportional to the radius, and therefore the rotational velocity decreases as the head moves from inside circumference to outside circumference. Thus, if reproduction is to continue from the first layer to the second layer, the head must be moved from the outside circumference to the inside circumference while simultaneously adjusting the rotational velocity of the medium.
When the information storage medium has two or more recording layers, there are two factors that must be considered when assigning the sector addresses. First, every address must be unique throughout the information storage medium. If the same address exists on the first and second layers, it is not possible to determine from the address alone whether the desired information is recorded on the first or second recording layer. Second, the addresses assigned to each layer should be easily convertible to an address on the first layer. This is because the address is the location information, and to move to the desired sector the movement distance must be calculated from the address. Particularly in a CLV information storage medium, the number of sectors per disk revolution is proportional to the radial position of the sector, and the sector number counted from the disk center is proportional to the surface area to the radial position of the sector. In other words, the groove number is in a square root relationship to the address of the sector counted from the disk center.
Apparatuses for reproducing a CLV disk must be able to calculate this square root in order to obtain the number of grooves the head must cross in order to be positioned to the desired sector. If converting the addresses on each layer to an address on the first layer is difficult, a different square root must be calculated for each layer.
Optical disk media standards generally define median and deviation values for the groove pitch and the radius of the groove closest to the inside circumference. Therefore, if the address at the inside circumference is indefinite relative to the radius of the inside circumference groove, the number of variables in the calculation obtaining the above square root increases. Thus, when the address at the inside circumference of each layer is indefinite, the time and tables required to calculate the square root increase. As a result, apparatuses for reproducing such disks incur cost increases from the square root tables required, and an increase in the processing time needed to calculate the square roots.
Conventionally, there has been proposed an optical disk having a plurality of recording layers to increase the recording capacity per one storage medium. Such an optical disk uses opposite side faces of the information storage medium, as in the case of the magnetic disk. One example is disclosed in Japanese Laid-open Patent Publication No. H2-103732. This reference discloses that the spiral track on the first side and that on the second side are in opposite direction for enabling smooth continuous play from the first side to the second side.
However, all the conventional optical disks of the two recording layer type has the recording surfaces facing in opposite directions, and both surfaces have the same reflectivity. Thus, one optical head is provided on each side, thus in total two optical heads in one reproducing apparatus. The optical head is an expensive device, because it generally includes a semi-conductor laser generator for the light source, optical devices for adjusting the light intensity, and an electromagnetic coil for adjusting the focusing point. Therefore, the reproducing apparatus used in connection with the conventional optical disk of the two recording layer type is normally a high cost apparatus.
Since there are two separate optical heads for the first and second sides of the optical disk, the first optical head for the first side surface may be located at the outer most track, whereas the second optical head for the second side surface may be located at the inner most track. Also, according to the recent development in the technology, which is called a jitter free reproduction technology, the reproduction can be properly carried out even when the disk rotation speed deviates from its proper speed. Therefore, in order to accomplish the smooth contiguous play from the first side to the second side, there is no limitation for the conventional two recording layer type optical disk to use a reproducing apparatus that moves the first head from an inside position to an outside position and then the second head from the outside position to the inside position, or vice versa, i.e., the first head from the outside position to the inside position and then the second head from the inside position to the outside position. It is possible that the first head may reproduce from the inside position to the outside position, and then the second head may reproduce from the inside position to the outside position.
Also, according to the conventional optical disk of the two recording layer type, since two separate optical heads are necessary it is possible to use the same addresses between the first side and the second side.
As understood from the above, according to the conventional two recording layer type optical disk, no consideration has been made to enable the smooth contiguous play from the first side to the second side using only one optical head. For the conventional two recording layer type optical disk, a plurality of optical heads are provided for enabling smooth contiguous play from the first side to the second side. Alternatively, one way to solve this drawback is to move the heads instantaneously from inside to outside, or vice versa, and at the same time change the rotational speed of the disk. However, from a practical view point, such an apparatus is not realized.
A problem with the conventional information storage medium thus described is that the groove formation and addresses are determined without considering contiguous reproduction across plural recording layers. As a result, a loss of performance and an increase in cost are incurred in apparatuses for reproducing such information storage media.